The invention is based on a method for measuring interference effects of a glass fiber transmission link as well as a transmission system according to the generic class of the independent claims. In Proceedings II, NOC '97, Antwerp, H. Bulow, pp. 65-72, the effects of polarization mode dispersion are described. Polarization mode dispersion occurs due to the birefringent effects in the glass fibers used for the transmission links. The light signal is divided into two components corresponding to the fast and the slow axes of the polarization states, whereby these two components are transmitted at different group velocities over the glass fiber link. The different group velocities of the two signal components cause interference and, in the general case, disturbance of the data information. The polarization mode dispersion effect is a statistical effect since the behavior of glass fibers varies due to their temperature, their state of strain, as well as due to aging effects. The change of the polarization mode dispersion is generally a slow process that takes place over longer periods. Consequently, it is not necessary to continuously measure the effects of polarization mode dispersion for a transmission link.
U.S. Pat. No. 5,473,457 discloses a method for measuring polarization mode dispersion. According to this method, the light signal is transmitted via a fiber and the polarization state is subsequently measured in a polarization controller. The signal is then supplied to a polarization maintaining fiber. The two polarization planes of the light are then separated in a device and coherently superimposed. The signal serves to calibrate the polarization controller. In this manner, the polarization state of the signal is actively influenced.